It is known to prepare 2-(4'-nitrophenyl)-propionic (I) acid by nitration of 2-phenyl-propionic acid or of the corresponding nitrile and by successive saponification.
However, these techniques show drawbacks due to the difficulty to supply the starting products and, especially also to the necessity to separate the nitration isomer products, an operation of not easy industrial realization.
Equally known is the carboxylation with CO.sub.2 of the para-nitroethylbenzene in dimethylformamide (DMF) in the presence of alkaline phenates. Nevertheless, this process, although offering some advantages from the point of view of the selectivity, yield and operational simplicity in comparison with the previously cited alternative processes, is not free from a number of drawbacks. In fact, the carboxylation reaction in DMF, besides the desired monocarboxylic (I) product, leads to consistent quantities, depending on the parametric conditions used, of the dicarboxylated product (paranitrophenylmethyl-malonic acid) better defined later on, with the introduction of a second-COOH group on the same carbon atom carrying the first group. This drawback makes it necessary to introduce a decarboxylation stage in order to eliminate the unwanted --COOH group, an operation that is, at any rate, possible according to known methods.
Moreover, the DMF solvent used is unstable under prolonged and/or repeated heating in an alkaline medium and, thus, the alkaline phenate necessary for the reaction, and which at the end of that reaction will be found in the form phenol, cannot be formed again in the presence of DMF which must thus be preliminarily separated.
In other words, the alkaline phenate must be prepared separately involving operational and economical problems of a certain purport. On the other hand, the actual possibilities of separation and recovery of the DMF from the phenol of the aqueous solutions at the end of the reaction, are practically inconsistent because of the great difficulties met in the extraction with solvents of the DMF from aqueous solutions. In fact, under those conditions only part of the DMF that is associated somehow with phenol is extracted, wherefore this latter cannot be in its turn used again.
Such difficulties bring with them the necessity of disposing and/or destroying great quantities of DMF solvent and phenol, with heavy economical and environmental implications.
Thus, an object of this invention is that of providing a process that is operationally accessible and economically convenient for the preparation of the 2-(4'-nitrophenyl)-propionic (I) acid.